1. Field of the Invention
The present invention relates to an ink composition for ink jet recording and a process for producing the same.
2. Background Art
Water-base ink compositions have been mainly used as an ink composition for an ink jet recording system from the viewpoint of odor, safety or the like. The ink compositions currently used is produced by dissolving various water-soluble dyes in water or a mixed solvent comprising water and a water-soluble organic solvent; and optionally adding various additives to the solution.
The following properties are required of an ink composition for ink jet recording:
(1) High-quality printing of letters and images on "plain" paper. PA1 (2) Good dryability of a print such that the print is not smudged when the print is rubbed immediately after printing. PA1 (3) Good filling property that provides stable jetting free from any of clogging, deviation of ink droplets from the flight path and dropouts. PA1 (4) Water resistance of the print. PA1 (5) Long-term storage stability of the ink. PA1 (6) Long-term reliability in terms of a property of the ink that does not attack the members with which the ink comes into contact.
In many cases, these properties, however, have a contradictory relationship with each other. Thus, it is considerably difficult to simultaneously satisfy these properties in a balanced manner. For example, when a penetrating agent for accelerating the penetration of the ink into the recording medium is added in order to improve the dryability of the print, the ink becomes likely to blot. Furthermore, the addition of a wetting agent in an excessive amount to the ink for the purpose of preventing clogging leads to a deterioration of the dryability of the print.
For this reason, no proposal capable of satisfying all the various property requirements has hitherto been made in the art. For example, although an ink composition containing 1,3-dimethyl-2-imidazolidinone, which is disclosed in Japanese Patent Laid-Open No. 71768/1980 (Japanese Patent Publication No. 31759/1982), has an excellent capability of preventing clogging, it cannot simultaneously satisfy all the above property requirements.
In particular, so far as the present inventors know, the ink composition disclosed in Japanese Patent Laid-Open No. 71768/1980 (Japanese Patent Publication No. 31759/1982) is unsatisfactory for the above properties (1), (2), (3) and (6). In particular, with respect to the property (2), even after a print is completely dried, contact or rubbing of the print on paper with a hand or a finger causes the paper to be smudged (hereinafter referred to as "dry to the touch"). This has been pointed out from the practical viewpoint. For the property (3), in some cases, even when discharge of bubbles generated in a recording head or ink passage is attempted, the bubbles cannot be discharged. The bubbles lead to the occurrence of dropouts or disturbance of the print. In addition, 1,3-dimethyl-2-imidazolidinone in the ink composition attacks materials constituting a recording head or ink passage and causes dissolution, swelling, cracking or roughening of the surface of the materials. (This property will be hereinafter referred to as an "attack property" of 1,3-dimethyl-2-imidazolidinone or an ink composition.) For the property (6), therefore, the ink composition cannot ensure the long-term stability particularly in a head made of plastic including a photosensitive resin.
In addition, Japanese Patent Laid-Open No. 144071/1980 (Japanese Patent Publication No. 52869/1986) and Japanese Patent Laid-Open No. 163168/1981 (Japanese Patent Publication No. 33315/1982) disclose inks similar to that disclosed in the Japanese Patent Laid-Open No. 71768/1980 (Japanese Patent Publication No. 31759/1982). However, these inks cannot satisfy all the above property requirements.